Field of the Invention
The present invention relates to an image forming apparatus, e.g., a printer, a copying machine, a multifunctional peripheral, or a fax machine.
Description of the Related Art
In recent years, there has been demanded particularly for an image forming apparatus for production printing to form an image on a sheet having an image quality satisfying, e.g., a higher level of color stability and graininess. In an electrophotographic image forming apparatus, images of three subtractive primary colors and black are independently formed on a plurality of photosensitive members, and those images of the respective colors are transferred onto an intermediate transfer member in a superimposed manner, to thereby form a full-color image on the intermediate transfer member. The image formed on the intermediate transfer member is transferred onto a sheet to form an image on the sheet. Image transfer from the photosensitive member onto the intermediate transfer member is referred to as “primary transfer”, and image transfer from the intermediate transfer member onto the sheet is referred to as “secondary transfer”. The intermediate transfer member is configured to perform the primary transfer and the secondary transfer while being rotated. Such an image forming apparatus employing a secondary transfer system can eliminate disturbance factors to be caused by the property of the sheet at the time of color superimposing as compared to a direct transfer system configured to directly superimpose and transfer images onto a sheet without using the intermediate transfer member. The image forming apparatus employing the secondary transfer system can thus stably form an image on a sheet.
In the image forming apparatus employing the secondary transfer system, the rotational speed (surface speed) of the intermediate transfer member is basically set substantially equal to the conveyance speed of the sheet. Hitherto, in some cases, the image forming apparatus employing this system changes those speeds to cause minute change in image length in the conveyance direction of the sheet. That is, the rotational speed of the intermediate transfer member and the conveyance speed of the sheet are adjusted to slightly change the magnification of the image in the conveyance direction. The image quality can be improved by adjusting the rotational speed of the intermediate transfer member and the conveyance speed of the sheet. For example, in order to reduce graininess in the case of a cardboard sheet or to improve image transfer performance in the case of an embossed sheet, the conveyance speed of the sheet is changed by about 1[%] with respect to the rotational speed of the intermediate transfer member. The difference between the surface speed of the intermediate transfer member and the conveyance speed of the sheet is hereinafter referred to as “speed difference”. Further, the conveyance direction of the sheet is a direction orthogonal to a main scanning direction in which laser light is scanned during image formation, and hence the conveyance direction is hereinafter sometimes referred to as a sub-scanning direction.
Secondary transfer is performed at a nip portion, which includes a drive roller around which the intermediate transfer member is looped and a secondary transfer roller, by nipping the intermediate transfer member and the sheet between the drive roller and the secondary transfer roller. In Japanese Patent Application Laid-open No. 2008-281931, there is disclosed an image forming apparatus in which the drive roller (intermediate transfer member) and the secondary transfer roller are respectively driven by independent drive sources, and the rotational speed of the secondary transfer roller is adjusted depending on the thickness of the sheet passing through the nip portion, to thereby suppress color misregistration.
When the speed difference is changed to improve the image quality as described above, the image is deformed, and the magnification in the sub-scanning direction is changed. When priority is given to image quality, the magnification of the image in the sub-scanning direction cannot be adjusted by adjusting the rotational speed of the secondary transfer roller. Therefore, in order to adjust the magnification in the sub-scanning direction without relying on the speed difference, it is necessary to perform image formation so as to cancel out the change in magnification in the sub-scanning direction.
Examples of the method of performing image formation so as to cancel out the change in magnification in the sub-scanning direction include an adjustment method of subjecting image data to be used for image formation to image processing to expand or contract the image data (digital sub-scanning scaling) and an adjustment method using a processing speed during image formation. When the adjustment method using the processing speed is performed, various image formation conditions are changed. Thus, a measurement image is formed on the intermediate transfer member, and the adjustment amount of the processing speed is feed-back controlled by measuring the size of the measurement image. This feed-back control is hereinafter referred to as “color registration”. Through color registration, a series of operations for feed-back is necessary, and hence a downtime is caused in the image forming processing. Therefore, the measurement image used for adjustment of the image size is required to be formed at an optimum timing.